Previously obtained evidence suggests that a 22,000 dalton protein of the cardiac sarcoplasmic reticulum (SR) mediates part of the mechanical response of the mammalian myocardium to catecholamines. Cyclic AMP-dependent phosphorylation of this protein is associated with an increased rate of calcium transport by isolated SR. An increased rate of removal of calcium from the contractile proteins as a result of increased Ca transport into the SR can account for the increased rate of relaxation and abbreviation of systole seen in the intact heart after epinephrine administration. Studies in isolated SR indicate various similarities between Ca uptake by the Ca pump and Ca release from calcium-loaded vesicles. In the forthcoming project year, the aims will be: (1) To determine the relationship of the Ca release measured in our in vitro studies to Ca release in the intact myocardium. (2) To obtain further evidence for or against the participation of the calcium pump protein in calcium release from SR. (3) To complete work on the chemical characterization of the 22,000 dalton phosphoprotein. Cyclic AMP-mediated phosphorylation of this protein, by virtue of its proposed interaction with the Ca pump protein, may thus not only increase the rate of cardiac relaxation but also affect the rate of tension development and total tension developed in the intact heart.